There are electrically operated vehicles with two electric motors provided in the drive train of said vehicles, which electric motors are controlled separately from one another. Therefore, the motor speeds and therefore also the electrical motor frequencies are not necessarily the same.
A drive device like that on which the present invention is based is illustrated in FIG. 1. The drive train in this case has, by way of example, two motors 1 and 2. Both motors are in this case in the form of three-phase motors. For actuation of the motors 1 and 2, two inverters 3 and 4 are used. The inverters 3 and 4 are fed via a DC link 5 from a battery 6 or another current source. In accordance with the equivalent circuit diagram illustrated in FIG. 1, the feedline to the inverter 3 has a resistance R1 and a line inductance L1. A coupling capacitance C1, which counts as part of the DC link 5, is connected upstream of the inverter 3. The feedline to the inverter 4 has an ohmic resistance R2 and a line inductance L2. A coupling capacitance C2 is connected upstream of the second inverter 4.
The DC link 5 is fed by a battery 6 with a battery inductance L3 and an internal resistance R3. However, for the further considerations, the ohmic resistances R1 to R3 are not taken into consideration. In addition, the battery inductance L3 is so high that it can be left out of the equation with respect to the line inductances L1 and L2 for the high dynamic range.
Owing to the above simplifications, the simplified circuit diagram of the DC link 5 illustrated in FIG. 2 results. FIG. 2 shows a series circuit comprising the capacitance C1 and an inductance L, wherein the capacitance C2 is connected in parallel with the series circuit. The inductance L corresponds to the sum of the line inductances L1 and L2. The current i1 leaving the first inverter 3 flows into the node between the capacitance C1 and the inductance L (cf. FIG. 1). Furthermore, a current i2 leaving the second inverter 4 flows into the node between the inductance L and the capacitance C2. A current iL flows through the inductance L.
The system shown in FIG. 2 has a resonant frequency, which can be seen from the system response in FIG. 3. This resonant frequency is, for example, 5.3 kHz if the line inductances are 1.5 pH and the inverter capacitances C1 and C2 are each 600 μF. In fact, the resonance in the system is damped by the ohmic resistances.
In the case under consideration, the inverters 3 and 4 generate pulse-width-modulated (PWM) actuation signals for the motors 1 and 2. The PWM frequency is 8 kHz, for example. Since this frequency is in the vicinity of the resonant frequency of the DC link, some spectral components of the PWM signals of the inverters 3 and 4 are also close to the resonant frequency. These signal components are amplified corresponding to the system response in FIG. 3. These undesired amplifications subject the DC link or input capacitances C1 and C2 to a load. They therefore need to be dimensioned correspondingly. In addition, considerable power losses arise owing to the undesired amplifications or peak values of magnification.
In the system shown in FIG. 1, substantially three oscillations with their harmonics occur during operation. Firstly, this is the frequency 8 kHz, at which the inverter is operated, for example. Secondly, the motors are operated, for example, at an average electrical speed of 120 Hz. Owing to non-ideal phenomena in the inverters and motors, a harmonic which corresponds to six times the frequency of the electrical motor frequency and is therefore 720 Hz in the selected example therefore results in the DC link. Furthermore, a third oscillation arises owing to the fact that the two motors have a speed difference of 20 Hz, for example.
In order to better understand the spectrum of the current iL flowing in the DC link, the following analysis is based on a dynamic model. Only the high frequencies are represented therein. The inverter operates at a fixed pulse frequency of 8 kHz. As a result of nonlinearities in the inverter/motor system, the frequency of 8 kHz acts as modulation carrier of the current signal.
The PWM frequency of an inverter can be noted asωINV=2·π·8000.
The electrical motor frequency is denoted by ωMOTOR.
The direct current for a motor then results as
                    ⁢                  i        ⁡                  (          t          )                    =              A        ⁢                                  ⁢                              sin            ⁡                          (                                                                    ω                    INV                                    ·                  t                                +                                  φ                  INV                                            )                                ·                      sin            ⁡                          (                                                6                  ·                                      ω                    MOTOR                                    ·                  t                                +                                  6                  ·                                      φ                    MOTOR                                                              )                                                      i      ⁡              (        t        )              =                  A        2            ⁡              [                              cos            ⁡                          (                                                                    ω                    INV                                    ·                  t                                -                                  6                  ·                                      ω                    MOTOR                                    ·                  t                                +                                  φ                  INV                                -                                  6                  ⁢                                      φ                    MOTOR                                                              )                                -                      cos            ⁡                          (                                                                    ω                    INV                                    ·                  t                                +                                  6                  ·                                      ω                    MOTOR                                    ·                  t                                +                                  φ                  INV                                +                                  6                  ·                                      φ                    MOTOR                                                              )                                      ]            
Accordingly, a motor generates two harmonics as shown in the above cosine arguments. These result from a multiplication of the sinusoidal values of the inverter frequencies and the motor frequencies.
Each of the motors in the drive system or the drive device generates such a current i1 and i2 as is also indicated in FIGS. 1 and 2. Both currents are summed in directionally dependent fashion so that the following total current results:i(t)=i1(t)−i2(t).
Finally, in each case two harmonics around the switching frequencies φINV 1 and φINV 2 of the inverters result:ω1=ωINV 1−6·ωMOTOR 1 ω2=ωINV 1−6·ωMOTOR 1 ω3=ωINV 2−6·ωMOTOR 2 ω4=ωINV 2−6·ωMOTOR 2 
The following simulation shows the currents iL occurring in the DC link in the time range. The frequencies are selected as follows:fINV 1=81 kHzfMOTOR 1=116.7 HzfINV 2=8 kHzfMOTOR 2=118.7 Hz
The frequencies of both inverters are in this case selected to be the same, therefore, with the result that there is a common switching frequency for the inverters. The electrical motor frequencies differ by 20 Hz.
FIGS. 4 and 5 show the simulated current signal iL on the basis of the above frequencies. In this case, FIG. 5 shows the segment between 0.04 s and 0.05 s shown in FIG. 4 on an enlarged scale.
There, the detailed structure of the signal and in particular the switching frequency of the inverters can also be seen.
The spectrum of the simulated signal is reproduced in FIG. 6. In said Figure, essential spectral components at 7300 Hz and 8700 Hz are shown. These spectral components originate from the motor speeds which produce a frequency of 700 Hz. Together with the PWM frequency of the inverters of 8 kHz, the following results: 8000 Hz−700 Hz=7300 Hz and 8000 Hz+700 Hz=8700 Hz.
As shown by the enlarged detail of the spectrum from FIG. 7, the small difference in the motor speeds results in two different spectral lines. One spectral line is actually 7300 Hz, while the other is 12 Hz below this, with a value of 7288 Hz.
If higher odd-order harmonics which are caused by the motor frequencies around the switching frequency are also taken into consideration in the simulation, the spectrum illustrated in FIG. 8 results. In this case, based on the frequency of 700 Hz which is brought about by the motor frequencies or the average motor frequency, odd harmonics result on the left-hand and right-hand side of the switching frequency of 8 kHz. In particular, the positive harmonics “+1st”, “+3rd”, “+5th” etc. arise on the right-hand side of the switching frequency. The negative harmonics “−1st”, “−3rd”, “−5th” etc. result on the left-hand side.
These frequencies are generally not a problem as long as they are not in the vicinity of the resonant frequency of the system or the DC link as shown in FIG. 3. If, however, the motor speeds change, the harmonic frequencies also change, and it may arise that one of these harmonic frequencies enters the resonance range and is amplified there excessively. The components of the drive device then need to withstand this excess current increase and, in addition, power losses arise as a result.